The modem daemon is started by the inetd process
if a client connects to the appropriate port as described below. The
modemd simply connects the socket handle with STDIN
and STDOUT of the cu command and lets
cu handle the actual modem device.

The existence of the modem daemon must be made known to the
inetd process by updating its configuration file,
usually /etc/inetd.conf like:

In order to make this work, an entry to
/etc/services needs to be added like:

modem 2006/tcp modemd

This associates a symbolic name with an explicit port, 2006 in the
example. The portnumber could be any number not already assigned to an
existing service. After these changes have been made, a signal must be
sent to the inetd process in order to let inetd
re-read and process its configuration file:

which would mean that a modem connected to
/dev/ttyS1 can be connected via port 5800 from
anywhere in the domain foo.org and from host 192.168.2.1. Any other
hosts are rejected. Pitfall: If you don't specify hosts then
ANY host will be allowed to connect. The serial
line settings are fixed and cannot be changed from the client side. You
can export several modems on a single port.
Masqdialer only locks the modem devices by
use of UUCP style lock files when they are actually in use thus
allowing other programs to take advantage of them.

At this time, only Windows client setups are described here. On the
client PC, a COM-port redirector for TCP/IP is required. The best
program for this purpose I have found is
DialOut/IP from Tactical Software for
Windows 3.1 and
Windows 95. (The Windows
3.1 version can be used under Windows
NT for 16-bit applications only. A 32-bit
version for Windows NT is due late summer
1997.)

DialOut/IP presents the shared modem on
a new virtual COM port that it adds to Windows.
This virtual COM port can be used by Windows
programs as if the shared modem is directly connected. Most client
applications (including Windows 95 dial-up
networking) accept this and work as if there were a real COM port and
modem, with the general exception being fax applications or any others
that need access to UART control lines. DialOut/IP
can be configured to provide telnet protocol processing, but that
feature applies to certain modem pool products and not to the
Linux setup described in this file. Note
that, despite its name, DialOut/IP
can be used also by applications that wait for incoming calls.

On http://www.tactical-sw.com/
there is a page for downloading a fully functional evaluation
version that times out in 1-2 weeks. Installation and configuration
is handled by a setup program, with installation details in the
README.TXT file. When you run
DialOut/IP, you enter the IP address
and port number of the shared modem.

DialOut/IP is a commercial product that
is licensed on a per-modem basis, that is, the price depends on the number
of modems that you are sharing. The license states that you can install
the software on any number of PC's that access the shared modems.

If you have only one modem for all your hosts in your local area
network, there is probably no reason to worry about security here.
However, if any one or more of the hosts in your LAN are connected to
the Internet by other means than using the modem we have just setup as
a modem server, then security considerations are required, otherwise
anybody can do a telnet your_host modem and dial
out long distance or even international calls at will.

I suggest to install and configure tcp-wrappers in order to protect
the modem server against unauthorized access.

I am using the setup as described in
The Server Side and
The Client Side to run
Quicken on
my Windows 95ThinkPad
and do home banking with the modem attached to my
Linux machine. The "modem" in my case is not
even a real modem, it is an emulated modem on an ISDN-So card.
Quicken just sees a COM port, it
doesn't know that the device attached to the COM port is actually at
the other end of my Ethernet LAN, nor does it know that it is not a
standard analog modem but an ISDN device which happens to understand
AT commands.

Since the first release of this document in June 1997
I have received many email messages related to the subject. In most of
the messages people were seeking more help to get the modem sharing
configured and running.

Recently I received an interesting feedback from
Karsten.Hilbert@gmx.net. Karsten pointed out that
although DialOut/IP may be a good client
program he'd like to have a
GPL'ed client. Karsten mentioned
the Software Bazaar
<http://visar.csustan.edu/bazaar/>
and he volunteered to put in an offer. Here is what Karsen wrote:

Date: Fri, 27 Aug 1999 17:46:39 +0200 (CEST)
From: Karsten Hilbert <med94ecz@studserv.uni-leipzig.de>
Reply-To: Karsten Hilbert <Karsten.Hilbert@gmx.net>
To: fb@baiti.net
Subject: Windows-Modemsharing-Howto
Hi !
The howto mentions DialOut/IP as a good tool to connect Windows
clients to a linux server sharing a modem.
I agree. However, it would be more attractive to have a free,
GPL'ed client, wouldn't it ? This I thought and decided to offer
some money for the implementation of such a client on the
Software Bazaar. If someone grabs the project I will pay him a
certain amount of money if it is completed and functional. After
that the client would be GPL'ed.
Now, I can only offer so much money :) But other people could
join in and offer some, too, thus increasing the incentive.
Wouldn't your howto be a perfect place to mention this
possibility ?
The Bazaar can be found at:
http://visar.csustan.edu/bazaar/
Thought I could mention this to you.
Karsten